Pigeon
Fever

Horses and other large animals worldwide are infected by Corynebacterium
pseudotuberculosis. There are also reports of infection in humans resulting from
exposure to infected sheep and horses. Pigeon Fever, the most common form of the
disease in horses, is characterized by external abscesses, primarily in the
pectoral area and ventral part of the abdomen. It is commonly diagnosed in
California and other arid regions of the western United States and appears to be
increasing in other, wetter areas of the country such as Kentucky and Colorado.

Treatment of this form of disease involves establishing drainage of the
abscess with lavage and disposal of the abscess contents to prevent further
contamination of the environment. Internal abscesses, which can be fatal if not
treated appropriately with antimicrobials, are less common than external
abscesses. A third form of disease, ulcerative lymphangitis, can result in
lameness and lymphatic damage (big leg) unless treated aggressively with
antimicrobials.

The portal of entry for Corynebacterium pseudotuberculosis, a potentially
soil-borne organism, is thought to be through abrasions or wounds in the skin
and mucous membranes. Insects such as the horn fly, house fly, stable fly, and
biting midge have been suggested as potential vectors in horses but have not
been confirmed experimentally. Using molecular techniques, a team of researchers
at University of California, Davis is determining the importance of these flies
in the transmission of the bacterium to horses.

Corynebacterium pseudotuberculosis is considered to survive for long periods
in the soil, and it appears endemic on most California farms and ranches. The
prevalence of disease is estimated at 10%, making the syndrome one of the most
common infectious bacterial diseases of horses in the state. Persistence of the
pathogen in the soil indicates that management techniques rather than
eradication efforts will be the best first step toward reducing disease
incidence. Further research is needed to better understand the epidemiology and
pathogenesis of this disease.

Controlling Pigeon Fever could be accomplished with a coordinated strategy
involving two complementary tactics: reducing the probability of transmission
and reducing the probability of infection should transmission occur. The disease
is seasonal, with peak incidence in the fall months, and the incidence
fluctuates from year to year within a site. It is not known whether these
patterns result from population growth of the pathogen during the warm months,
from population fluctuations of the potential vectors, or both. While vaccines
are commercially available for sheep and goats, none are available for horses.
Administration of bacterins or toxoids offers excellent protection in sheep
(more than 90% against experimental challenge). There are strain differences
between bacteria infecting horses and small ruminants, so that research is
needed to develop a product for use in horses. Dr. Spier has been working with
autogenous bacterins/toxoids with the intention of developing a vaccine for
horses. Dr. Janet Foley, a UC Davis researcher, is also investigating the strain
homogeneity of C. pseudotuberculosis collected from horses in California,
Kentucky, and Colorado.